1. Field of the Invention
The present invention relates generally to optical communication in a Wavelength Division Multiplexing (WDM) system, and in particular, to a device for suppressing the transient effect caused by the increased or decreased number of connections in the optical system.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) combines signals at different wavelengths to travel through the same fiber. The WDM has become important as it can multiply the transmission capacity of a single fiber without the installation of a new cable.
In fiber amplifiers, an optical fiber is the amplifying medium where stimulated emission takes place, yield a stronger optical signal. The WDM system is provided with an optical fiber amplifier for amplifying an optical signal without the need for opto-electrical conversion. The optical fiber amplifier amplifies an optical signal relying on the amplification of a rare-earth (e.g., erbium) element doped fiber, which is disclosed in detail in U.S. Pat. Nos. 5,218,608, 5,245,690, and 5,467,218. However, the conventional amplifiers have some drawbacks of a decreased output level and deteriorated S/N (signal-to-noise) ratio. Thus, it is necessary to monitor the number of channels, the output power level, and the SNR (Signal-to-Noise Ratio) of channels during the transmission of light signals in the WDM system.
Accordingly, the transient effect of remaining channels resulting from the on/off or add/drop of channels in the fiber amplifier affects the reliable operation of the system and also deteriorates the transmission quality of an optical signal. For example, in the event that 8-channels are dropped in the 16-channel transmitting WDM system, the optical fiber amplifier increases the output power per channel above a reference output power level in order to maintain the total output power of 16 channels; thus, the transient phenomenon generates overshoot or undershoot of the output power in the system.
The transient effect described in the preceding paragraphs can be suppressed by modifying the optical structure of the optical fiber amplifier, or controlling the light intensity of the pump laser diode. The former optical approach is effective in suppressing the transient effect but has drawbacks in that the optical system and the circuit for controlling the optical amplifier are complicated. In the latter approach, the transient effect of a single channel or a few channels can be suppressed, but it is not effective where a large number of channels is changed abruptly.
It is, therefore, an object of the present invention to provide a device for suppressing the transient effect of an optical fiber amplifier caused by a channel on/off or channel add/drop in order to prevent the deterioration of the transmission quality of an optical signal in the WDM system.
It is another object of the present invention to provide a device for suppressing the transient effect of an optical fiber amplifier by appropriately controlling the gain of the optical fiber amplifier when a large number of channels is changed in a WDM system.
Accordingly, a device for suppressing the transient effect of an optical fiber amplifier in a WDM system includes an input sensor that converts an input optical signal to an electrical signal; a transient effect suppresser for holding the electrical signal received from the input sensor for a predetermined time and for outputting the delayed signal in the form of pulses that are proportional to the variation of the number of channels; a voltage comparator for comparing the predetermined reference voltages with the amplitude of the pulses received from the transient effect suppresser and for outputting a trigger signal when the channel number is changed; a pump laser diode driver for supplying a driving voltage to a pump laser diode in response to the received pump indication signal; and, a controller for outputting the pump indication signal corresponding to the changed channel number in response to the trigger signal.